The drying of such sheet-like materials is carried out in most cases by means of a predominantly convective heat transfer in the form of cross-flowing of heated air. The sheets in this case, often also distributed over a plurality of tiers, are guided through the drier by means of feed or transporting devices such as roller conveyors or screen belts.
According to the prior art, drying plants are operated in most cases in circulating air mode. The drying air in this case is repeatedly directed onto the sheets and reheated after each contact. The air is increasingly enriched with moisture in this way, only a small portion of the drying air being discharged to the environment as exhaust air in order to discharge moisture and flue gases to the environment.
A characteristic feature of different types of construction of drier forms the type of air guiding over the material to be dried. The air can basically be guided onto the sheet in the form of cross aeration, longitudinal aeration, or so-called impingement jet aeration.
In the case of cross aeration, the drying air is guided over the material to be dried from the side, transversely to the feed direction of the sheet-like material. Since the drying air is increasingly cooled during its path over the material to be dried, different drying speeds are consequently created across the width. Therefore, this method is not used in the case of sensitive materials such as sandwich-type drywall sheets.
In the case of longitudinal aeration, the drying air travels over a long path along the longitudinal axis of the drier, flows over the sheet in the process, dries this and cools down to a considerable extent as a result. The drying air, therefore, at low temperatures, can be discharged in an energetically especially favorable manner close to the dew point of the drying air. For heating fresh air by means of a heat exchanger, condensation heat can then be purposefully utilized.
In the case of impingement jet aeration, the drying air is fed from the side of the drying plant in air lines, in so-called nozzle boxes, and, via air-exit nozzles, is blown perpendicularly onto the surface of the material to be dried. From there, this air flows to the opposite side of the drying plant.
Driers operating in accordance with a similar construction are distributed on a worldwide basis in the meantime. Counted among their advantages is the fact that as a result of the construction consisting of a multiplicity of relatively short drying chambers, which in each case can be individually aerated and heated, the desired drying temperature and the climate over the length of the drier can be freely selected. Therefore, the drying conditions can be adapted to the requirements of the material to be dried. The drier, moreover, can be controlled in an excellent manner, e.g. during product changes.
As a result of the good heat transfer during the impingement jet inflow, such driers can be of a considerably shorter construction than comparable driers which are exposed to flow with longitudinal aeration.
By adjusting the nozzle box inclination, moreover, an extremely uniform drying across the width of the material to be dried can be achieved.
The exhaust air of each chamber is individually discharged and collected. Since chambers with process-induced high drying temperatures are also ranked among these, an altogether high exhaust air temperature results. Also, by using a heat exchanger, the condensation heat contained within the exhaust air humidity can hardly be meaningfully utilized.
Such a plant is described in DE 19 46 696 A1 under the title of a method and a device for accelerated drying of drywall sheets. The printed publication deals with the description of the drying chamber which is designed so that a heat yield which is as high as possible and drying which is as uniform as possible across the width of the material to be dried is ensured. Measures for reducing the energy consumption are not mentioned, however.
A two-stage drying method and a drying plant is known from DE 26 13 512 A1, upon which is based the object of modifying or supplementing the as known per se two-stage drying method so that especially drywall sheets or materials with similar properties can be economically dried according to this method.
In the case of the two-stage drying method, the second drying stage, by interposing a heat exchanger, is heated from the exhaust air of the first drying stage. The sheets are to be dried in the first drying stage at high temperature and with high air humidity, and in the second drying stage are to be dried at relatively low temperature and with low air humidity. The first stage in this case is longitudinally aerated, and the second stage is cross aerated. Impingement jet aeration is not used. A very low consumption can certainly be realized with this type of construction. On account of the indirect heating of the second stage, the temperature level is very low, however. A low drying capacity and a high consumption of feed power result accordingly. The drier has not, therefore, been able to be put through in practice.
Furthermore, a method for drying sheets and a corresponding drier is known from DE 43 26 877 C1.
Based on the method according to DE 26 13 512, a method with the lowest possible primary and secondary energy consumption is described. In particular, the primary energy used is to be minimized as far as possible by utilizing the waste heat and also the condensation heat of the exhaust air without increasing the need for secondary energy by circulating large air mass flows. This object is achieved in this case by the exhaust air of stage A being directed in stage B through a heat exchanger, which is arranged in the tiers of the drier, and by the drying air, at low temperature and with low air humidity, being guided in counterflow to the exhaust air of stage A.
Stage B, which is responsible for the cooling of the exhaust air, in this case, however, has no impingement jet aeration and as a result of the indirect heating the drying capacity of stage B is quite low.